As a general rule, superfinishing machines of the type noted are designed such that a workpiece is centerlessly received between two rollers rotating in the same direction. The rollers are somewhat obliquely positioned with respect to one another, in terms of their longitudinal axes, so that they impart to the workpiece not only a rotational movement but also, at the same time, a feeding movement in the direction of its longitudinal axis, along the gap between the rollers. While the workpieces are being rotated and moved along in the feeding direction by the rollers, honing stones are seated on the workpieces. The honing stones are made to vibrate parallel to the axial direction of the workpieces by a vibrating mechanism. As a result, the roundness and surface finish of the workpieces are improved.
In order to eliminate the out-of-roundness and roughness of the workpieces still present after preliminary machining, which is generally a grinding operation, a certain amount of workpiece material must first be removed. Only then is the finish improved by reducing the peak-to-valley height of the surface. Furthermore, a high machining or throughput speed and short machine idle periods are desirable. For these reasons, a plurality of honing stones of varying coarseness or fineness are disposed in succession in the feed direction above the gap between the rollers. The first honing stones beneath which the workpiece travels are relatively coarse. They remove a relatively large amount of material in order to improve the roundness. Subsequently, the workpiece travels along beneath finer honing stones; these stones remove less material and effect a substantial improvement in the finish, until the peak-to-valley height is extremely low.
The honing stones must be changed relatively frequently, because they cannot be arbitrarily long, since as very high stones, they tend to vibrate, and since they cannot be fabricated with a uniform structure. The operation of changing the stones, which involves the removal of the worn honing stones and the mounting of new, unworn stones, takes a relatively long time. The changing operation must be performed relatively frequently, on account of the varying structure of the stones and the different tasks to which they are put, causing them to wear down at varying rates. The idle periods of the machine increase in cases where one operator monitors several machines. Also, under the conditions of restricted space prevailing in a superfinishing machine of this kind mistakes occur. For example, it is difficult to prevent one type of stone being mistaken for another, or for incorrectly securing the stones.